The present invention relates generally to electronic circuit assemblies including land grid array-type devices, and more particularly to biasing assemblies for an electronic circuit assembly that includes a land grid array-type device and a heat sink.
Printed circuit boards are generally formed of a rigid dielectric material which is printed with a predetermined pattern of an electrical conductor. Printed circuit boards may be electrically connected to one or more land grid array-type devices such as an application specific integrated circuit (ASIC) or a flexible printed circuit having an array of electrically conductive pads thereon. In order to electrically connect a land grid array-type device to a printed circuit board, an electrical connector or xe2x80x9csocketxe2x80x9d may be disposed therebetween which has an array of electrically conductive pads on each side thereof. The electrically conductive pads may be constructed from an elastomeric material. The pads on one side of the connector abut with the pads on the land grid array-type device, and the pads on the other side of the connector abut with the electrically conductive array on the printed circuit board.
In order to maintain electrical connection between a land grid array-type device and a printed circuit board, the device and the board must be compressed together, with the electrical connector therebetween. Such an assembly 10 is shown in FIG. 1. The surfaces 12, 14, respectively, of the device 20 (an ASIC being shown in this figure and the board 22 that the electrical connector 24 is in between must each be flat to within a few mils of an inch. When pads 26, 28 (shown greatly enlarged for illustrative purposes) on an electrical connector 24 are compressed between a land grid array-type device 20 and a printed circuit board 22, these pads 26 (especially elastomeric ones) act as miniature springs, exerting forces xe2x80x9cF0xe2x80x9d opposing the compression of the device 20 and the board 22. Existing large-area connector arrays generate large forces between the printed circuit board and the device being attached to the board. These forces are often large enough to deflect the printed circuit board outside of the flatness requirements. Thus, in addition to needing a relatively large compressive force to maintain contact between the device, the connector and the board, a backing plate 30, FIG. 1, is required to support the printed circuit board 22 and maintain the flatness of the front surface 14 thereof. As shown in FIG. 1, such a backing plate 30 is usually positioned on the back side 16 of the printed circuit board 22, opposite the electrical connector 24 and land grid array-type device 20. A second backing plate 32, which may be part of a heat sink (not shown) or the like, may be positioned adjacent to the land grid array-type device 20.
As shown in FIG. 1, a biasing assembly 34 such as springs 36, 38 is generally required to maintain a large, relatively constant force xe2x80x9cF1xe2x80x9d on the board, connector and device. Such a biasing assembly 34 is usually placed on the top side 14 of the printed circuit board 22, adjacent to the second backing plate 32, as shown in FIG. 1. In general, with a linear spring, the force xe2x80x9cFxe2x80x9d provided by a spring is directly proportional to the spring constant xe2x80x9cKxe2x80x9d multiplied by the linear deflection xe2x80x9cXxe2x80x9d (F=KX). A spring having a low spring constant xe2x80x9cKxe2x80x9d is most desirable in this application in order to keep the spring force as consistent as possible. Specifically, manufacturing tolerances can vary among different installations. In addition, changes in environmental conditions such as temperature and creep of various components may cause the spring to deflect. Because of F=KX, a large spring constant xe2x80x9cKxe2x80x9d multiplied by even a small change in deflection xe2x80x9cXxe2x80x9d of the spring would produce a relatively large fluctuation in the force xe2x80x9cFxe2x80x9d provided by the spring.
Since a large force xe2x80x9cFxe2x80x9d is required and a low spring constant xe2x80x9cKxe2x80x9d is most desirable, the linear deflection xe2x80x9cXxe2x80x9d of any linear spring used in this application must be large. Furthermore, since a spring with more coils deflects a greater total distance than the same type of spring with fewer coils, a coil spring used in this application must be relatively long. Specifically with reference to FIG. 1, in order to provide a sufficient force xe2x80x9cF1xe2x80x9d to oppose the large forces xe2x80x9cF0xe2x80x9d generated by the pads 26 on the electrical connector 24, the length xe2x80x9cL1xe2x80x9d of each spring 36, 38 (shown compressed) must be relatively large. In today""s small, densely-packed computers and electronics, the distance required for such springs 36, 38 may not be available on the top side 14 of a printed circuit board 22. Even if such a distance is available, providing a more compact biasing assembly is more desirable.
Oftentimes, a heat sink must be installed over an electrical component such as a land grid array-type device in order to dissipate heat generated by the device. A heat sink is typically constructed from a heat conductive material such as, for example, aluminum, magnesium, or copper, and has a base portion with a plurality of cooling fins attached thereto. The base portion typically draws heat from the electrical component and then spreads and transfers the heat to the cooling fins. The base of a heat sink is typically positioned directly adjacent to the land grid array-type device, possibly with thermal interface material therebetween.
Adding a heat sink to an electronic circuit assembly presents the further problem of providing a thermal connection between a heat sink and a land grid array-type device while also providing an electrical connection between the land grid array-type device, an electrical connector, and a printed circuit board. A biasing assembly (e.g., 34 described above) is generally required to provide a compressive force in order to maintain the necessary connections between the heat sink, land grid array-type device, electrical connector, and printed circuit board. However, during installation thereof, the force must be applied as uniformly as possible to avoid rocking and possibly damaging the land grid array-type device, electrical connector, and printed circuit board.
Thus, it is an object of the present invention to provide a backing plate assembly which includes a biasing assembly to provide a constant compressive force on a printed circuit board, electrical connector and land grid array-type device.
It is also an object of the present invention to provide a heat sink assembly which includes a biasing assembly to provide a constant compressive force on a heat sink, printed circuit board, electrical connector and land grid array-type device.
It is a further object of the present invention to provide a biasing assembly having a relatively low spring constant which provides a relatively large compressive force on a heat sink (if present), printed circuit board, electrical connector, and land grid array-type device, yet does not require a relatively large distance on the top or bottom side of the printed circuit board.
It is also an object of the present invention to provide a spring-loaded backing plate assembly as a single, compact unit positioned on the back side of a printed circuit board.
It is a further object of the present invention to use a simple, relatively low-cost leaf spring assembly, rather than a coil spring assembly, as the biasing assembly in a spring-loaded backing plate or heat sink assembly.
It is a further object of the present invention to provide a spring-loaded backing plate or heat sink assembly which provides a predetermined, constant force upon every installation thereof in a circuit assembly.
It is a further object of the present invention to provide a method for installing a spring-loaded heat sink assembly on a circuit assembly which applies biasing force in a uniform manner and prevents rocking of the components.
It is a further object of the present invention to provide a tool for installing a spring-loaded heat sink assembly which releases the biasing force in a uniform manner and prevents rocking of the components.
In accordance with these and other objects, the present invention is directed to a spring-loaded heat sink assembly for a circuit assembly. The assembly may comprise a heat sink having a base portion, a plurality of cooling fins, and an uppermost surface on the cooling fins. At least one leaf spring may be positioned adjacent to the uppermost surface of the heat sink. The assembly may further comprise a load plate positioned adjacent to the leaf spring(s). The load plate may comprise a plurality of elongate shafts fixedly attached thereto. The elongate shafts removably extend through the leaf spring and the heat sink and at least partially through the circuit assembly. Each of the elongate shafts has an open channel extending therethrough. The assembly may further comprise a plurality of fasteners extending through the open channel of the elongate shafts and at least partially through the circuit assembly. The fasteners removably connect the spring-loaded heat sink assembly to the circuit assembly.
The present invention is also directed to a spring-loaded heat sink assembly and circuit assembly, whereby the circuit assembly has at least a printed circuit board, an electrical connector, and a land grid array-type device. The assembly may comprise a heat sink having a base portion in thermal contact with the land grid array-type device and a plurality of cooling fins. At least one leaf spring may be positioned adjacent to the cooling fins. The assembly may further comprise a load plate positioned adjacent to the leaf spring(s). A plurality of elongate shafts may extend through the heat sink and leaf spring(s) from the load plate to at least the land grid array-type device. The assembly may further comprise a plurality of fasteners extending through an open channel in the elongate shafts, thereby removably connecting the spring-loaded heat sink assembly to the circuit assembly. The circuit assembly may further comprise an electromagnetic interference frame mounted on the printed circuit board and a backing plate. If so, the elongate shafts may extend from the load plate to the electromagnetic interference frame. The fasteners may extend through the electromagnetic interference frame and the printed circuit board and be removably attached to the backing plate.
The present invention is also directed to an installation tool for attaching a spring-loaded heat sink assembly to a circuit assembly. The installation tool may comprise an upper portion, a first side portion, and a second side portion. The tool may further comprise an attachment pin removably inserted through the first side portion and the second side portion extending from and separated by the upper portion. The tool may further comprise a lower plate and an actuation device extending through the upper portion and contacting the upper surface of the lower plate. The actuation device is adjustable in order to move the lower plate relative to the upper portion of the tool. The tool may further comprise at least one fastener movably connecting the lower plate to the upper portion.
The present invention is also directed to a method for installing a spring-loaded heat sink assembly on a circuit assembly using an installation tool. The method may comprise the initial step of assembling a heat sink, at least one leaf spring, and load plate. The next step may involve mounting an installation tool on the heat sink assembly. The next step may involve lowering an actuation device on the installation tool in order to compress the leaf spring(s). The next step may involve positioning the heat sink assembly and attached installation tool on the circuit assembly. The next step may involve inserting fasteners through the heat sink assembly and at least partially through the circuit assembly in order to attach the heat sink assembly to the circuit assembly. A final step may involve removing the attachment pin and installation tool from the heat sink assembly.